This application claims the priority of Japanese Patent Application No. 11-210100 filed on Jul. 26, 1999, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an infrared lens; and, in particular, to a bright wide-angle infrared lens employed in an infrared imaging system in a wavelength band of 8 to 12 xcexcm.
2. Description of the Prior Art
Known as detectors employed in infrared imaging systems are those of cooling type such as those in hybrid mode (using InSb or HgCdTe) and Schottky barrier form (using PtSi), and non-cooling type using a thermopile or microbolometer. As characteristics of these types, the cooling type is problematic in that it is expensive and hard to be made compact though it can achieve a high sensitivity, whereas the non-cooling type has a temperature resolution lower than that of the cooling type though being able to achieve a lower cost and compactness. Therefore, in either case, it is desirable that bright lenses be used in optical systems for collecting the heat radiated from an object, i.e., infrared rays, and forming an image on a detector surface in order to improve noise equivalent temperature difference (hereinafter referred to as NETD). In particular, for enhancing the sensitivity in the non-cooling type, bright lenses having a very small F number are required.
While crystal materials such as Ge, Si, and ZnSe are employed as optical materials transmitting infrared rays therethrough, these materials are quite expensive, whereby they increases the cost greatly if the number of lenses is large. Also, depending on the materials, the ratio of light absorption is so much that the decrease in transmissivity becomes problematic when lenses are made thicker or the number of lenses increases, whereby NETD may deteriorate greatly. Hence, there is a strong demand for infrared lenses to cut down their number, and there is also a demand for compact lenses in order to respond to smaller dimensions of cameras themselves.
Further, as the number of pixels increases in detectors, sufficient imaging performances and wider field of view are required.
As conventional techniques having solved such problems to a certain extent, those disclosed in the following publications have been known:
U.K. Patent Publication No. 1,345,505 (Fno=0.80, 2xcfx89=35xc2x0, three-element lens configuration);
U.S. Pat. No. 4,030,805 (Fno=0.59, 2xcfx89=16.5xc2x0, four-element lens configuration);
Japanese Unexamined Patent Publication No. 52-85834 (Fno=1.0, 2xcfx89=9.2xc2x0, three-element lens configuration);
Japanese Unexamined Patent Publication No. 52-86344 (Fno=0.9, 2xcfx89=19xc2x0, three-element lens configuration);
Japanese Unexamined Patent Publication No. 52-100247 (Fno=0.8, 2xcfx89=18xc2x0, four-element lens configuration);
Japanese Unexamined Patent Publication No. 62-5211 (Fno=0.9, 2xcfx89=10.2xc2x0, three-element lens configuration); and
Japanese Unexamined Patent Publication No. 62-30208 (Fno=0.8, 2xcfx89=10.2xc2x0, three-element lens configuration).
Though the techniques disclosed in the above-mentioned publications satisfy some of the above-mentioned demands, none of them satisfies all the demands. For example, those yielding a small F number with a brightness and wide field of view may yield very large coma and curvature of field, thus being problematic in terms of performances.
In view of the foregoing circumstances, it is an object of the present invention to provide a bright wide-angle infrared lens, in a compact configuration of three elements, exhibiting favorable performances though having a very small F number and a wide field of view.
The present invention provides a bright wide-angle infrared lens comprising, successively from an object side, a first lens made of a meniscus lens having a positive refracting power with a convex surface directed onto the object side, a second lens made of a meniscus lens having a negative refracting power with a convex surface directed onto the object side, and a third lens having a positive refracting power with a convex surface directed onto the object side, at least one surface of the first lens being formed aspheric.
Preferably, the infrared lens satisfies the following conditional expressions (1), (2), and (3):
1.50 less than f1-2/fxe2x80x83xe2x80x83(1)
0.45 less than f3/f less than 0.65xe2x80x83xe2x80x83(2)
0.55 less than D2-3/f less than 0.80xe2x80x83xe2x80x83(3)
where
f is the focal length of the whole system;
f1-2 is the composite focal length of the first and second lenses;
f3 is the focal length of the third lens; and
D2-3 is the air space between the second and third lenses.
Preferably, the image-surface-side surface of the first lens and the object-side surface of the third lens are formed aspheric.
Preferably, all of the first, second, and third lenses are formed from germanium and are used for a wavelength band of 8 to 12 xcexcm.